The present invention relates to an eye examining instrument, particularly for examining and photographing the interior of an eye.
In order to use eye examining instruments, such as eye fundus cameras, for mass examinations to prevent adult diseases, the picture angles of these instruments have been made wider and wider. This permits inspection and photography of a large portion of the eye at one time so that a number of persons can be examined in a short period. However, if damage or disease is suspected from the first examination or photographic inspection, an eye fundus camera is still needed for producing a highly magnified picture.
Eye fundus cameras operate by illuminating an eye fundus when the latter is coaxial with the camera's objective lens. Under these circumstances, a part of the illuminating light is reflected by the cornea of the eye and strikes the film. This causes undesirable flare on the film.
In a known practical large eye fundus camera an apertured mirror is placed between the objective lens and the image forming lens. A black spot is provided at a position conjugate with the cornea between the mirror and the light source, so that its' image is formed on the cornea. The eye fundus illumination light is permitted in at the edge of the black spot's shadow. The light reflected on the eye fundus passes through the shadow formed on the cornea so as to separate the illumination light from the reflected light. Thus the reflected light passes through the aperture in the mirror, and is led to the image forming lens. On the other hand, part of the illumination light is reflected on the objective lens. The latter is shaped such that the light reflected on it is condensed on the black spot provided in the lens. However, it is difficult and time consuming to manufacture an objective lens with a special shape, and its picture angle as well as its efficiency are restricted. In accordance with the Japanese Patent Publication No. Sho 44-8406, a black spot is provided between the light source and the aperture mirror so as to interrupt the light beam reflected on the objective lens surface in order to avoid the above mentioned difficulty.
Another problem occurs when the picture field angle of the eye fundus camera is enlarged from 30.degree. to 45.degree.. That is, when the picture field angle is narrow the image of the black spot formed on the cornea has a long tail directed toward the eye fundus so that the front and the rear surfaces of the lens lie in this shadow. Thus no reflection take place. When, however, the incident angle of the illumination light becomes larger due to the increase of the field angle, the shadow of the black spot becomes shorter such that a part of the illumination light is reflected on the surface of the lens so that a flare occurs on the picture. In accordance with U.S. Pat. No. 3,851,954, a black spot is provided between the light source and the aperture mirror so as to eliminate the light reflected on the lens.
Further, in accordance with U.S. Pat. No. 4,102,563, other undesirable light due to the lens is eliminated. This undesirable light arises because, the liquid in the lens is not always transparent so that the illuminating light is dispersed. Further, the illuminating light is dispersed on the rear surface of the lens, so that a flare appears like mist over a wider range than that due to the reflected light which appears comparatively clear. As the counter-measure against the above, the range on the lens through which the photographic light (light beam striking the film through the objective lens, the aperture in the mirror and the image forming lens) passes is covered with a black spot.
When a black spot is provided, the illumination light incident on the eye fundus is decreased by the amount of light interrupted with the image of the spot. However, for an eye fundus camera of a non pupil dilation system with focus adjustment by means of infrared light, no pupil dilation means is used, and a picture is taken in a natural state with a flash. However, in extreme cases such as that of an old man whose pupil can not be dilated sufficiently, even in a dark chamber the amount of the photographic light is often too small. Consequently, an under-exposure occurs in an instrument whose flash intensity is predetermined.
Further, in the case of the fluorescent fundus angiography an exciter filter is provided in the illumination light, while a barrier filter is provided in the photographic light. Thus the light transmittance is substantially decreased, and it becomes necessary to increase the retina illumination more than in ordinary photography.